Motor vehicles are equipped with seat belts, usually in the form of a combined shoulder and pelvis belt, but occasionally also in the form merely of a shoulder belt in order, in the event of a collision of the vehicle with an obstacle or another vehicle, to hold back its occupant uniformly with the vehicle by using the braking distance which is formed essentially by the bonnet which projects forward, and so protect him from serious injuries which are usually the result of the striking of the occupant against more or less rigid parts of the bodywork or the steering wheel. As seat belts are frequently not applied because of negligence, particularly in short-distance and town traffic, where driving is at sufficiently low speeds to make a maximum protective effect possible, so-called passive seat belt systems have been produced which ensure automatic application of belts when the door of the vehicle is closeed.